From P2P Foundation

Directory

1. Open BSC:OpenBSC is a GSM network in a box software, implementing the minimal necessary parts to build a small, self-contained GSM network.
2. Open BTS
3. OsmocomBB: By using OsmocomBB on a compatible phone, you are able to make and receive phone calls, send and receive SMS, etc. based on Free Software only.

More Information

1. P2P Telephony